1. Field of the Invention
The present invention relates to an organic light emitting device, and more particularly, to an organic light emitting device that has increased light extraction efficiency and improved color reproducibility due to an optical path control layer.
2. Description of the Related Art
Organic light emitting devices (OLEDs) are display devices that display an image using light emitted in processes of radiative recombination of excitons which are formed by combining holes supplied from an anode electrode with electrons supplied from a cathode electrode in the organic light emitting layer. Due to high display characteristics such as wide viewing angle, short response time, thin size, low manufacturing costs, and high contrast, the OLEDs are expected to be the next generation of flat panel display device.
OLEDs can be classified into passive matrix (PM) type OLEDs and active matrix (AM) type OLEDs according to the driving method, and can also be classified into bottom emission type OLEDs and top emission type OLEDs according to the direction of light emission from the organic light emitting layer.
OLEDs display red, green, blue, or white color according to material selection for forming the organic light emitting layer. Of the full color technique of OLED apparatuses, the white color OLED apparatus has an advantage of displaying a high resolution image using a large size mother glass in OLED apparatuses and a color filter technique of conventional liquid crystal displays (LCDs).
An OLED includes a transparent electrode, a light emitting layer, an electron injection layer, and a reflection electrode, which are sequentially stacked on a substrate. The OLED may additionally include layers such as a hole injection layer, a hole transport layer, and an electron transport layer. When voltage is applied between the transparent electrode and the reflection electrode, electrons and holes are combined, and a spectrum of light having a predetermined wavelength is generated from the light emitting layer. A portion of the light emitted from the light emitting layer is emitted to the outside through the transparent electrode. At this point, light having an incident angle greater than a critical angle at an interface of each of the layers cannot be emitted to the outside.
An optical extraction efficiency is a ratio of light extracted to the outside with respect to a total amount of light emitted from the light emitting layer. Most of the photons are trapped in an OLED due to internal reflection or extracted to laterals, and thus, cannot contribute to the optical output of light. Accordingly, the optical extraction efficiency of the OLED is merely approximately 20%. Therefore, research for increasing the optical extraction efficiency is necessary.
In an AM type 12.5-inch OLED display, which that was disclosed in Article No. 29.5L and was displayed at the Society for Information Display (SID) in 2004 by SONY, the color purity and efficiency were improved by employing a cavity effect in a top emission type OLED. RGB pixels having different thicknesses of indium tin oxide (ITO) are formed on a reflection anode. However, in this technique, the manufacturing process is complicated because of the ITO layers having different thicknesses.
In U.S. patent publication No. 2005/0194896 and Korean Patent Publication No. 10-2004-0030359, an optical scattering layer is included to increase optical extraction efficiency. In this technique, a scattering center having a minute structure is employed in a matrix of the optical scattering layer, and the optical extraction efficiency is increased through the scattering of light.
In the published in JAP Vol. 93, 2003, pp. 19 by Takahiro Shiga of Toyota Central Research Center, a technique that can increase optical extraction efficiency of a bottom emission type white OLED was disclosed. In this report, a multi-wavelength micro-cavity structure having two modes using two dielectric mirrors obtained by forming a multi-layered thin film of ITO/SiO2/TiO2 on a glass substrate has been reported. However, this technique has a complicated manufacturing process, because the multi-layered thin film must be formed on the glass substrate.